This invention relates to a guard ring structure for a bipolar semiconductor device.
Guard structures, generally in the form of rings, are used to control potential gradients at the periphery of bipolar semiconductor devices. In a conventional guard ring, a metal equipotential ring extends around the periphery of the device. The equipotential ring is typically formed of aluminium that is deposited on a silicon oxide insulator which overlies an implanted or diffused guard region formed in a substrate of the device. In a bipolar device, an emitter guard region is diffused or implanted into the substrate at the outer edge of the device and is electrically connected to the equipotential ring to form the guard ring. The guard ring is laterally spaced from the active region of the semiconductor surface.
The guard ring is used to control potential gradients at the periphery of the device.
An electrical connection to a bipolar semiconductor device incorporating a guard ring may be made by depositing a bond on the surface of the semiconductor. The bond can, however, often be misaligned and can overlie the guard ring. This can create an electrical connection between the guard ring and the bond, causing a permanent or intermittent short circuit which will lead to device failure or reliability problems.
The equipotential ring is sometimes covered with a layer of dielectric compound. However, as the equipotential ring is typically of aluminium, which can only withstand temperatures up to approximately 500xc2x0 C., the dielectric layer cannot be densified, as this would require temperatures of the order of 800-900xc2x0 C. The dielectric layer thus remains relatively soft and fragile and is easily damaged. The dielectric layer does not therefore provide very good protection against short circuiting as a result of bond misalignment.
It is an object of the present invention to obviate or mitigate such problems with the prior art.
According to the present invention there is provided a bipolar semiconductor device guard structure, comprising a guard region formed in a substrate of the device and extending adjacent a peripheral portion of the device, an insulating layer formed on the substrate between the peripheral portion of the device and the guard region, a polysilicon layer formed on the insulating layer, a layer of densified dielectric covering the polysilicon layer, and at least one electrical interconnection between the polysilicon layer and the guard region.
Thus, use of polysilicon rather than aluminium makes it possible to protect the guard ring structure with a densified dielectric. The risk of short circuiting between the active surface of the device and the guard ring is thus greatly reduced.
The polysilicon layer may overlap an outer edge of the insulating layer. The guard region may be diffused or implanted into the substrate. The polysilicon may define all or part of a closed ring of conductive material of for example rectangular shape, and four electrical interconnections may be provided, one at each corner of the rectangular shape.